Records of the effects of atmospheric pressure fluctuations on subsurface gas date back to early observations of weather patterns. The flow of air throughout cave entrances, particularly during rapid barometric pressure changes, are recorded throughout the world. The atmospheric pressure effect and its implications for gas transport is mentioned in a 1904 monograph describing the release of carbonic acid from soil and its replacement with oxygen from the subsurface in response to atmospheric pressure changes. See E. Buckingham, Contributions in Our Knowledge of the Aeration of Soils, U.S. Department of Agriculture Bureau of Soils Bulletin. This monograph describes a soil cleaning effect that occurs because of barometric changes and suggests that the response of subsurface air pressure changes are damped and time delayed with respect to surface pressure changes. The monograph further suggests that the amount of damping and delay increases with further depth into the soil.
From a study of atmospheric pressure, it is known that oscillations in barometric pressure are naturally periodic and sinusoidal in nature. Such oscillations are also diurnal, resulting from both daily temperature changes in the earth's atmosphere and the passage of weather fronts. Daily pressure variations resulting from temperature changes average about 5 millibars (one millibar is approximately one thousandth of an atmosphere), while those due to weather front passages can be 25 or more millibars.
As the barometric pressure rises (such as on a downward oscillatory cycle), a gradient is imposed on the earthen soil gas which drives fresh surface air into the near surface of the soil. As the pressure drops (such as on an upward oscillatory cycle), soil gas very near the soil surface vents upward from the surface into the atmosphere. This cyclic phenomena is observable with perforated wells which are formed in the ground through bore holes. Such wells are observed to "breathe" in that they inhale ambient air from the surface and exhale soil gas during cyclic barometric pressure changes. This well "breathing" results primarily from pressure differentials that occur between the soil air pressure near the open end of a well and the surrounding atmospheric barometric pressure.
Gas trapped in the earthen soil is capable of movement in the soil under several conditions. The total movement of soil gas in the earthen soil depends primarily on the magnitude and frequency of the barometric pressure oscillations, the soil gas permeability and the depth (or distance) to an impermeable boundary. Such an impermeable boundary can include a localized water table, bedrock or extensive layers of low permeability material (such as caliche or clay). Since the daily change in atmospheric pressure is small (typically 0.5 percent), the overall net soil gas displacement during the daily cycle is also small (with an estimated range of centimeters to meters). Furthermore, the daily oscillations in atmospheric pressure always return to a mean value. Therefore, over a period of time, no significant net soil gas displacement occurs in the subsurface due to advective forces alone.
The dominant characteristic defining the magnitude and frequency of soil gas movement is due to the approximately 5 millibar variation which results daily from heating and cooling of the atmosphere. In higher permeable soil, the gas velocities will be greater for a given atmospheric pressure variation. Peak gas velocity also increases as the depth to an impermeable layer increases. The peak soil gas velocity, as determined by analytically modeling the soil gas response, will range from 0.2 to 0.8 meters/day for a typical range in permeability (1 to 10 Darcies) and depths to an impermeable layer of 50 meters or more. Under natural conditions, this oscillatory movement results in no net flow because it always returns to its mean value.
Soil gas movement is also triggered by composition of the source contaminant. For example, volatile contamination sources typically exist as liquid deposits in the soil. The liquid evaporates and results in an initial vapor concentration in the air immediately adjacent to the liquid. Contaminant vapor diffuses away from the contaminant source at a rate governed by the diffusion constant of the contaminant in soil gas, the porosity of the soil, the soil tortuosity (deviation from a straight line path through the soil pores), absorption of the contaminant into the soil, the ability of the adjacent soil to supply adequate thermal energy to vaporize the liquid contaminant and other related effects. Under these situations, analysis of soil gas diffusion is analogous to heat transfer, where gas concentration is substituted for temperature and an effective soil diffusivity is substituted for the heat transfer constant.
The diffusion of soil gas is also caused by density of the contaminant which might be sufficient to drive the gas vapors towards the water table. Such "density induced" transport of soil gas are induced by temperature and contaminant concentration gradients in the soil gas. An effective remediation system must counteract these forces.
While there are numerous strategies for removing soil gas contaminants from the earthen soil, two techniques receive the most attention: active vapor extraction and passive vapor extraction.
Active vapor extraction involves withdrawing soil gas from contaminated areas of the vadose zone and then treated for removal of volatile organic compounds ("VOCs"). Unlike passive vapor extraction, however, active vapor extraction techniques require the disturbance of the earthen soil, such as digging or boring a well into the subsurface soil. These bore hole wells typically employ a blower, compressor and/or a vacuum device in an attempt to remove contaminated vapor from the well site to the surface for treatment and/or discharge. The typical active vapor extraction system also includes a water knockout tank to remove entrained water, an air cooler to reduce the temperature of the gas stream temperature downstream of the blower, canisters for absorption of VOCs, and a process control system. Typically, active vapor extraction techniques rely on horizontal transport of subterranean gas into the well, which is then vented out vertically within the well by the assistance of the blower and/or vacuum device. These techniques are featured in U.S. Pat. Nos. 5,288,169 to Neeper, 5,271,693 to Johnson et al, 5,076,727 to Johnson et al, and U.S. Pat. No. 5,249,888 to Braithwaite et al. Another example of removing contaminated soil gas from the ground involves the application of heat in the soil by injecting heated posts into the ground, again illustrating disturbance of the soil. These techniques are illustrated in U.S. Pat. Nos. 5,209,604 to Chou and 5,244,310 to Johnson.
Sites having a high concentration of contamination are perfect candidates for active vapor extraction. Active vapor extraction could be used to extract soil gas from areas of high volatile organic compound ("VOC") concentrations and within zones exhibiting high vapor phase permeability. However, for many other sites, residual contamination exists which could not practically or completely be removed by the active vapor extraction technology. These circumstances result in sites which are slightly contaminated, but by state and federal regulation, the contamination must still be monitored, controlled and/or removed.
Further, while active vapor extraction techniques may achieve partial or complete removal of contaminated soil gas quickly, they are also very expensive. In contrast, passive vapor extraction systems are useful for applications not requiring immediate attention and cheaper solutions. For example, a majority of Department of Energy sites are contaminated with VOCs. In many instances, the contamination has not reached the local water table, does not pose an immediate health hazard and therefore, is not considered a high priority problem. Nevertheless, these sites will ultimately require remediation of some type. Precipitating or enhancing the natural breathing process creates the potential for increased removal rates of VOCs from the vadose zone.
Passive vapor extraction technology, such as disclosed in the present invention, exploits the natural flow of air through the subsurface as a means of mobilizing volatile contaminants in the vadose zone toward collection points at the surface for treatment. The flow of air through the subsurface is primarily a function of the difference between the barometric pressure, the air pressure in the soil and the permeability of the soil. The permeability of the soil controls the rate at which air flows into and from the soil. Low permeability soils will exhibit low flow rates and require a longer period of time to equilibrate with a change in barometric pressure. Changes in soil depth and stratigraphy also affect pressure differences and flow rates.
Passive vapor extraction appears viable as a technology that can complement active vapor extraction under certain conditions. While active vapor extraction is useful to extract soil gas from areas of high VOC concentration and within zones exhibiting high vapor phase permeability, passive vapor extraction is useful in areas of lower VOC concentration and in relatively impermeable soils where extraction rates are limited by gaseous diffusion. A primary advantage of passive vapor extraction is lower capital and operating costs. The low cost of passive vapor extraction allows for many small passive vapor extraction systems to be installed on individual wells within a contaminated site and to be operated for extended periods of time. This allows for remediation of sites in which soil-gas transport is limited by diffusion.
Accordingly, it is an object of the present invention to provide a passive noninvasive soil remediation apparatus and method which uses an impermeable surface seal and unidirectional venting features to rectify and remediate volatile organic compound contaminated soils in the vadose zone of earthen soil.
It is an object of the present invention to provide a noninvasive soil remediation method and apparatus of maximized geometric configuration over a predetermined area to precipitate and enhance vertical displacement of soil gas in the subterranean soil.
It is an object of the present invention to provide a noninvasive soil remediation method and apparatus of a certain plenum size, buffer zone configuration, plume depth, and geologic setting (depth to impermeable zone) specifically adapted to displace soil gas in a preferred coordinate direction.
It is a further object of the present invention to provide a noninvasive soil remediation method for controlling displacement of soil gas within the earthen subsurface which impedes and minimizes downward movement of such gas or atmospheric air and stimulates upward gas movement.
It is also an object of the present invention to provide a noninvasive soil remediation method and apparatus including a surface seal, a plenum and an extraction vent valve placed above a contaminated soil gas plume and adapted to rectify soil gas displacement from traveling in a downward direction from the soil surface, and allowing such gas to travel in an upward direction.
It is also an object of the present invention to provide a soil remediation method and apparatus and method capable of rectifying soil gas displacement from traveling in a downward direction from the soil surface during periods of rising atmospheric pressure, and allowing such gas to travel in an upward direction during periods of atmospheric barometric pressure.